Method of Seismic Surveying

ABSTRACT

A method enables the time required to complete a seismic survey and the noises recorded in the seismic data to be reduced. One aspect of the method includes actuating a first vibrator group to start a first sweep at time T 0 ; and a second vibrator group to start a second sweep at time T 1 , wherein T 0 &lt;T 1 &lt;T 0 +S 1 +L, where S 1  is a sweep time of the first vibrator group and L is a listening time; wherein the time between the first sweep and the second sweep is T 1 −T 0 ≧((n−1)*f 1 *S 1 )/(n*(f 1 −f 0 )) where n is a natural number, f 0  is the lower frequency limit of the vibrator sweep and f 1  is the upper frequency limit of the vibrator sweep.

RELATED APPLICATIONS

The present US patent application, attorney docket No. 57.0371-US-CON2is a Continuation application of the co-pending U.S. patent applicationSer. No. 11/548, 521, filed on 11 Oct. 2006, attorney docket No.57.0371-US-CON1, now a U.S. Pat. No. 7,376,046, issued on May 20, 2008,by the same inventor with the same title and assigned to the sameassignee, which is currently pending.

U.S. patent application Ser. No. 11/548, 521 is a Continuationapplication of U.S. patent application Ser. No. 11/299,410, attorneydocket No. 57.0371-US-DIV, filed on 12 Dec. 2005, by the same inventorwith the same title and assigned to the same assignee, which iscurrently pending.

U.S. patent application Ser. No. 11/299,410, attorney docket No.56.0371-US-DIV is a Divisional application of U.S. patent applicationSer. No. 10/403,135, attorney docket number 57.0371-US, filed on 31 Mar.2003, by the same inventor with the same title and assigned to the sameassignee, which is now U.S. Pat. No. 7,050,356, issued on 23 May 2006.

U.S. patent application Ser. No. 10/403,135, attorney docket No.57.0371-US claims priority from a GB Patent Application No. 0207995.2,filed on 6 Apr. 2002, by the same inventor with the same title andassigned to the same assignee, which is now GB Patent No. 2 387 226,issued on 10 Aug. 2005.

FIELD OF THE INVENTION

The present invention relates to a method of seismic surveying. Inparticular, it relates to a method of seismic surveying in which two ormore groups of vibrator sources emit seismic energy in such a way thattheir sweep times overlap with one another.

BACKGROUND OF THE INVENTION

The principle of seismic surveying is that a source of seismic energy iscaused to produce seismic energy that propagates downwardly through theearth. The downwardly-propagating seismic energy is reflected by one ormore geological structures within the earth that act as partialreflectors of seismic energy. The reflected seismic energy is detectedby one or more sensors (generally referred to as “receivers”). It ispossible to obtain information about the geological structure of theearth from seismic energy that undergoes reflection within the earth andis subsequently acquired at the receivers.

In practice, a seismic surveying arrangement comprises an array ofsources of seismic energy. This is because it is necessary to generatesufficient energy to illuminate structures deep within the earth, and asingle seismic source generally cannot do this.

Sources of seismic energy are known which emit seismic energy at morethan one frequency. Examples of such seismic sources are vibratorsources, which emit seismic energy in a frequency range of, for example,from 5 or 10 Hz to 100 Hz. When such a vibrator source is actuated,seismic energy is emitted over a finite time period, and the frequencyof the emitted energy changes during the period over which seismicenergy is emitted. For example, the frequency of the emitted energy mayincrease monotonically during the period over which seismic energy isemitted. The process of operating a vibrator source of seismic energy tocause emission of seismic energy over the frequency range of thevibrator will be referred to herein as “sweeping” the vibrator, and thestep of initiating a vibrator sweep will be referred to as “actuating”the vibrator. Each emission of seismic energy from a vibrator is knownas a “shot”. The time period over which seismic energy is emitted by thevibrator source will be referred to as the “sweep time”, and the “sweeprate” is the rate at which the frequency changes over the sweep time (alinear sweep rate is generally used in practice).

A seismic vibrator source for use on land consists generally of abaseplate in contact with the ground. Seismic energy is transmitted intothe ground by applying a vibratory force to the plate, and this is doneby applying a control waveform known as a “pilot sweep” to the vibratorcontrol mechanism. The pilot sweep is generally a constant amplitudeswept frequency signal, although it tapers off at each end to allow theamplitude of the vibration to be ramped up and down at the start andfinish of the sweep respectively. In practice the waveform applied tothe ground by the plate is not exactly the same as the pilot waveform;in particular, as well as applying a force at the desired frequency atany particular time (known as the “fundamental frequency”), the vibratoralso applies a force at integer multiples of the fundamental frequency(known as “harmonics”).

Marine vibrator sources of seismic energy are also known. They are againswept so as to emit seismic energy over a range of frequencies.

When a seismic vibrator source is actuated to emit seismic energy, theseismic energy incident on a receiver is recorded for a pre-determinedperiod from the start of the sweep time of the source. The time from theend of the sweep time to the end of the recording period is generallyknown as the “listening time”, and data is acquired at a receiver fromthe start of the sweep time to the end of the listening time. The dataacquired at a receiver in consequence of actuation of a source is thenprocessed, for example by cross-correlating the acquired data with thepilot sweep of the source to produce a record that is the length of thelistening time.

FIG. 1 is a schematic illustration of the process of a conventionalseismic survey that uses an array of land vibrator sources. At time T0,one seismic source in the source array is actuated to start its sweep.In this example, the vibrator sweep time has a duration S, and thefrequency of seismic energy emitted by the vibrator increasesmonotonically from a frequency f₀ at time T0 to a frequency f₁(f₁>f₀) atthe conclusion of the sweep (at time T0+S). The sweep time is followedby the listening time, so that the overall time of the process ofactuating and sweeping the source and listening at a receiver forseismic energy is S+L, where L is the duration of the listening time.

In a conventional seismic survey the sources are actuated such that areceiver will receive seismic energy from only one source in any givenlistening period. The minimum delay between the start of two vibratorsweeps in such a survey is therefore the sum of the sweep time S and thelistening time L. The listening time L is made sufficiently large thatall seismic energy required at a receiver in a particular listeningperiod was emitted during the sweep time immediately preceding thatlistening period.

The conventional seismic surveying process has the disadvantage that itcan be slow, owing to the need for the minimum time delay between thestarts of two vibrator sweeps to be the sum of the sweep time and thelistening time. One known attempt to reduce the time required to carryout a seismic survey is the “slip-sweep” acquisition technique. In theslip-sweep technique the minimum time delay between the starts of twosubsequent vibrator sweeps is only the listening time, not the sum ofthe sweep time and the listening time. The record length aftercross-correlation is the length of the listening time.

The slip-sweep technique is illustrated in FIG. 2. As in the method ofFIG. 1, one seismic source in the source array is actuated to start itssweep at time T0, the vibrator sweep time has a duration S, and thesweep period is followed by a listening time L. The time T1 at which asecond source is actuated to start its sweep is not however required tosatisfy T1>T0+S+L, but is only required to satisfy T1>T0+L. Since theminimum time delay between actuation of two sources in the slip-sweeptechnique is only the listening time, not the sum of the sweep time andthe listening time, the slip-sweep technique allows the time to completea seismic survey to be reduced. It has the disadvantage, however, thatharmonics of the fundamental frequency generated by one vibrator arepresent on the seismogram recorded by one or more preceding vibrators.

A further known surveying technique is the technique of “simultaneousshooting”. In the simultaneous shooting method two or more seismicsources disposed at respective shot locations are actuated to starttheir sweeps at the same time. The seismic energy acquired at a receiverwill therefore contain events arising from seismic energy emitted by allsources. In order to allow the events corresponding to each source to beseparated out from one another, each vibrator must sweep, at its shotlocation, at least as many times as there are vibrators in the group,and the recorded data are then manipulated algebraically to separate theevents corresponding to each source. Typically each vibrator will sweepfor the same length of time, at the same sweep rate, and over the samefrequency range, but the phase relationship between vibrators changesfrom one record to another. In the case of a group of two vibrators, forexample, one suitable scheme would be for the two vibrators to sweep inphase during the first record and to sweep 180° out of phase during thesecond record. The mean of the two signals acquired by a receiver givesthe signal arising at that receiver from actuation of one vibrator, andhalf the difference of the two signals acquired by a receiver gives thesignal arising at that receiver from actuation of the other vibrator.

SUMMARY OF THE INVENTION

A first aspect of the present invention provides a method of seismicsurveying comprising the steps of: actuating the or each vibrator in afirst vibrator group at time T0; and actuating the or each vibrator in asecond vibrator group at time T1, where

T0<T1<T0+S1+L where S1 is the sweep time of the first vibrator group andL is the time during which data is acquired at a receiver; wherein atleast one of the first vibrator group and the second vibrator groupcomprises at least two vibrators.

The present invention provides a method of seismic surveying thatcombines the known simultaneous acquisition technique and the knownslip-sweep acquisition technique. It makes use of the simultaneousacquisition technique in that at least one of the groups of vibratorscontains two or more vibrators, but the different groups are swept usinga slip-sweep technique. The present invention provides a reduction inthe time required to carry out a seismic survey compared to the timerequired by a conventional slip-sweep technique.

The invention may be applied to a single vibrator group, in which casethe second vibrator group is the first vibrator group, or the first andsecond vibrator groups may be distinct.

In a preferred embodiment the method comprises the step of actuating theor each vibrator group at least as many times as there are vibrators inthe respective group and such that the contribution from each vibratorin a group may be determined by algebraic operation on receiver recordsof sweeps made by the respective group.

In a preferred embodiment T1−T0>(n−1)S1f₁/n(f₁−f₀), where n is a naturalnumber, f₀ is the lower frequency limit of the vibrator sweep and f₁ isthe upper frequency limit of the vibrator sweep. This enables the noisein the acquired data arising from the m^(th) harmonic of the fundamentalfrequency to be estimated for all m≦n.

In a preferred embodiment the method comprises the steps of: actuatingthe or each vibrator in the first vibrator group at time T2, whereT1<T2<T1+S2+L where S2 is the sweep time of the second vibrator group;and actuating the or each vibrator in the second vibrator group at timeT3 where T2<T3<T2+S+L; the first vibrator group is different from thesecond vibrator group; and T3−T2≠T1−T0.

This embodiment allows a further noise reduction technique to beapplied. The varying time delay between a shot of the first group andthe shot of a second group means that harmonic noise will occur atdifferent times in the records of the two shots. The noise may thereforeeliminated by appropriately summing the two shot records, on theassumption that each shot record contains the same signal.

A second aspect of the present invention provides a method of seismicsurveying comprising the steps of: actuating a first vibrator at timeT0; actuating a second vibrator different from the first vibrator attime T1, where T0<T1<T0+S1+L where S1 is the sweep time of the firstvibrator and L is the time during which data is acquired at a receiver;actuating the first vibrator at time T2, where T1<T2<T1+S2+L where S2 isthe sweep time of the second vibrator; and actuating the second vibratorat time T3 where T2<T3<T2+S1+L and where T3−T2≠T1−T0.

In a preferred embodiment T3−T2>T1−T0.

A third aspect of the present invention provides a seismic surveyingarrangement comprising: a plurality of vibrator sources; and a controlmeans adapted to actuate the or each vibrator in a first vibrator groupat time T0 and to actuate the or each vibrator in a second vibratorgroup at time T1, where T0<T1<T0+S1+L where S1 is the sweep time of thefirst vibrator group and L is the time during which data is acquired ata receiver.

A fourth aspect of the present invention provides a seismic surveyingarrangement comprising: a plurality of vibrator sources; and a controlmeans adapted to: (a) actuate a first vibrator at time T0; (b) actuate asecond vibrator different from the first vibrator at time T1, whereT0<T1<T0+S1+L where S1 is the sweep time of the first vibrator and L isthe time during which data is acquired at a receiver; (c) actuate thefirst vibrator at time T2, where T1<T2<T1+S2+L where S2 is the sweeptime of the second vibrator; and (d) actuate the second vibrator at timeT3 where T2<T3<T2+S1+L and where T3−T2≠T1−T0.

In a preferred embodiment the control means comprises a programmabledata processor.

A fifth aspect of the invention provides a medium containing a programfor the data processor of a seismic surveying arrangement as definedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described byway of illustrative examples with reference to the accompanying figuresin which:

FIG. 1 is a schematic timing diagram of a conventional seismic survey;

FIG. 2 is a schematic timing diagram of a conventional slip-sweepseismic survey;

FIG. 3 is a schematic timing diagram of a seismic survey according to afirst embodiment of the present invention;

FIG. 4 is a block schematic flow diagram of one embodiment of thepresent invention;

FIG. 5 is a schematic illustration of raw seismic data;

FIG. 6 illustrates the result of processing the raw seismic data of FIG.5 according to a first method of the present invention;

FIG. 7 illustrates the results of processing the seismic data of FIG. 5according to a second method of the present invention;

FIG. 8 shows the result of processing the seismic data of FIG. 5according to the first and second methods of the invention;

FIG. 9( a) is a schematic diagram of a seismic surveying arrangementaccording to the present invention; and

FIG. 9( b) is a schematic diagram of a control means of the seismicsurveying arrangement of FIG. 9( a).

DETAILED DESCRIPTION OF THE INVENTION

The operation of a seismic surveying arrangement according to a firstembodiment of the present invention is illustrated schematically in FIG.3, which is a timing diagram of the method. This method enables the timetaken to complete a seismic survey to be reduced compared to theconventional slip-sweep method described above.

The method assumes that the seismic survey has an array of seismicvibrator sources. In order to carry out the method, the vibrators aregrouped into two or more groups, with one group of vibrators beingoperable independently of the or each other group of vibrators. Thegrouping may be a physical grouping, for example with the vibratorsbeing arranged in a two-dimensional array with each row or columnconstituting a group. Alternatively the grouping may be a notionalgrouping, in which vibrators in an array are notionally divided into twoor more groups. It is not necessary for each group to have the samenumber of vibrators, but at least one group is required to contain twoor more vibrators.

At time T0 a first group of vibrators is actuated. That is, eachvibrator in the first group starts its sweep at time T0. All vibratorsin the first group will have the same sweep time. (For each group, allvibrators in the group will have the sweep time. In principle differentgroups could have different sweep times, although it is usual for allgroups to have the same sweep time as one another.) If the sweep time ofthe vibrators in the first group is S1, then the sweep period of thefirst vibrator group lasts from time T0 to time T0+S1. The sweep periodfor the first group is then followed by a listening time, which has aduration L and so concludes at time T0+S1+L.

A second group of vibrators is actuated to start their sweeps at timeT1. The time T1 is before the end of the listening period of the firstvibrator group—that is,

T1<T0+S1+L. The group of vibrators actuated at time T1 is referred to asa second vibrator group for convenience, but the second vibrator groupeither could be different from, or could be the same as, the first groupof vibrators. The sweep period of the second vibrator group has aduration S2 (which may be equal to S1) and so extends from T1 to T1+S2,and the listening period then extends to T1+S2+L (the two groups willhave the same listening time). The time delay between the start of thesweep of the first vibrator group and the start of the sweep of thesecond vibrator group must exceed the listening time (and if the samegroup is swept twice in succession then the delay must be greater thanthe sweep time of that group).

FIG. 3 shows only the first sweep of the first group and the first sweepof the second group. However, the method of the invention requires thateach group of vibrators is swept at least as many times are there arevibrators in that group, in a sequence from which the contribution fromeach vibrator to the seismic data acquired at a receiver can bedetermined, for example algebraically.

In general, there will be M groups of vibrators, with N vibrators ineach group. If each group makes K shots, where K≧N, there will be asequence of MK shots. For example, if there are two groups of threevibrators (M=2, N=3) each group must make at least 3 shots (K≧3). Ifeach group makes exactly three shots, one possible sequence would be:group 1; group 2; group 1; group 2; group 1; group 2. Somedifferentiation between the sweeps, for example a phase change, isrequired to allow the records to be algebraically manipulated to isolatethe contribution of each vibrator. (In principle, if a group were tomake more shots than there are vibrators in the group (that is, if K>N)some of the shots could be identical provided that there were at least Nindependent shots.)

As another example, consider a single group of 4 (M=1, N=4). In thiscase the lowest value of K is 4, so the group must be swept at least 4times and the shortest possible sequence is: group 1; group 1; group 1;group 1.

As another example, consider six vibrators arranged in three groups eachof two vibrators (M=3, N=2). In this case the lowest value of K is 2. Ifeach group makes exactly two shots, one possible sequence would be:group 1; group 2; group 3; group 1; group 2; group 3.

In this method of the invention, at least one of the shots takes placewithin time S+L of the start of the preceding shot. Preferably, as manyas possible of the shots start less than S+L after the previous shot. Itwill be noted that the very last shot will not contain slip-sweepnoise—for example, in the simple example of shooting with just twovibrators, the second vibrator would shoot within the listening time ofthe first vibrator (that is, within time S+L of the start of the firstshot). Both vibrators would then be moved to their next shot points sothat the shot of the second vibrator would not have any slip-sweep noiseon it.

In the above examples each group of vibrators contains the same numberof vibrators. The invention is not limited to this, and the groups neednot all contain the same number of vibrators (although that there mustbe a group containing more than one vibrator). If the groups do notcontain the same number of vibrators, each group must be swept at leastone more time than there are vibrators in that group.

FIG. 3 indicates that vibrators in a third vibrator group (which couldbe the first or second group) are actuated to start their sweeps at timeT2 that satisfies T2<T1+S2+L, where S2 is the sweep time of the secondgroup of vibrators. It would, however, be possible for the third groupto be actuated at a time T2>T1+S2+L since it is necessary for only oneshot to start within S+L of the start of the previous shot.

Once each vibrator group has been actuated a sufficient number of timesthe vibrators may be moved to different positions to allow a freshsurvey to be carried out.

The response at each receiver owing to the seismic energy generated byeach individual vibrator may be calculated from the seismic energyacquired at the receiver(s) by using any suitable technique. Onesuitable technique is disclosed in UK patent application No. 2 359 363.In principle the response could be calculated immediately the data havebeen acquired, but it is more usual for the data to be stored, forexample on magnetic tape or disc, for subsequent processing away fromthe survey location.

One problem with slip-sweep acquisition is that the data obtained in thelistening period following one sweep will contain harmonic noise thatarises from the subsequent sweep. It is desirable to eliminate thisharmonic noise during processing of the data. A further embodiment ofthe present invention provides a method for estimating the harmonicnoise in data acquired in a listening period from the acquired data.

The harmonic noise in data acquired in one listening period is theearth's response to the harmonic output from the subsequent sweep. Theearth's response for the harmonics of the subsequent shot is the same asthe earth's response to the fundamental sweep of the shot associatedwith the listening period. Vibrator deconvolution theory is based onknowing the content of the fundamental sweep as this provides ameasurement of the earth's response. If it is possible to estimate alsothe harmonic output of the vibrator, then it is possible to estimate theharmonic contribution to the preceding shot by convolving the earth'sresponse, as determined from the content of the fundamental sweep, withthe harmonic output of the vibrator. Once the harmonic contribution tothe preceding shot has been found in this way, it is possible tosubtract it from the recorded data.

In principle, it would be possible to use measurements on the seismicenergy emitted at the vibrator to estimate the harmonic output of thevibrator. However, the effects of the non-linearity of the earth'sbehaviour near the vibrator can mean that the harmonic noise at thereceiver differs from the harmonic noise that would be estimated frommeasurements on the vibrator. Estimating the harmonic output from thedata itself is therefore a more reliable method.

One embodiment of a method for eliminating harmonic noise from dataacquired in the simultaneous slip-sweep acquisition technique or FIG. 3will now be described with reference to FIG. 4.

Initially, at step 1, vibrator sources are grouped into two or moregroups. This grouping step may be a physical grouping step, for exampleduring the deployment of the vibrators. Alternatively it may consist ofdefining notional groups in a vibrator array. At least one of the groupscontains two or more vibrators.

At step 2, simultaneous slip-sweep data are acquired using anacquisition technique of the type described generally with respect toFIG. 3.

At step 3, the impulse response is calculated for each group, from thepositions of the vibrators in that group. This step may be carried outin a conventional manner using cross-correlation and an inversion matrixthat is constant with frequency, or it may alternatively be done in themanner described in GB-A-2 359 363.

Next, at step 4, the acquired data are cross-correlated with a harmonicfrequency sweep. The sweep that is used in this cross-correlation stepis essentially the same as the fundamental sweep of the vibrator, exceptthat it has a sweep rate that is an integer multiple of the sweep rateof the fundamental sweep. Thus, the sweep used in the cross-correlationstep would have twice the sweep rate of the fundamental vibrator sweepif it is desired to remove the second harmonic, would have three timesthe sweep rate of the fundamental sweep for removing the third harmonic,and so on. It is not necessary for the sweep used in thecross-correlation step to extend above the upper frequency limit of thefundamental sweep—and in any case the upper frequency should not exceedthe Nyquist frequency for the sampling—so a taper is applied to theharmonic sweep after a frequency near the upper frequency of thefundamental sweep. For each harmonic, the same harmonic sweep may beused in the cross-correlation step for all receivers and for all shots.

The effect of the cross-correlation step is to concentrate in time theearth's response to one harmonic. The earth's response to otherharmonics and to the fundamental frequency are spread out in time. Inparticular, although the earth's response to the fundamental sweepfrequency will still be the main contributor to the acquired data, theeffect of the cross-correlation step is that it will now arrive at alater time in the records.

At step 5, the cross-correlated data is Fourier transformed over a timewindow that includes the main first arrival due to the desired harmonic,but which ends before the earth's response to the fundamental vibratorfrequency is seen or (in the case of estimating higher harmonics) whenthe earth's response to a higher amplitude harmonic is seen. The timewindow length used in this step may vary from one receiver to another.For each frequency and receiver, the result of the Fourier transform isa vector G_(n) having length N, where N is the number of shots. Thisvector is the early part of the earth's response to the chosen vibratorharmonic.

At step 6, the impulse response for each receiver and vibrator positionis treated with a Fourier transform of the same length as in step 5. TheFourier transform is carried out over the same time window, or aslightly shorter time window, than the Fourier transform in step 5. Theresults of this Fourier transform step are a vector R_(n) for eachreceiver and frequency, with R_(n) having a length M where M is thenumber of vibrator positions.

At step 7, the matrix G_(n)R*_(n) is calculated for each frequency andreceiver. R*_(n) is the complex conjugate of the transpose of R_(n).This yields an N×M matrix. This matrix is then averaged over receiversto give the cross-correlation matrix

G_(n)R*_(n)

. A receiver-based normalisation may be applied in this averagingprocess, to allow for the varying signal amplitude at differentreceivers.

At step 8 the M×M matrix R_(n)R*_(n) is calculated for each frequencyand receiver. This is then averaged over receivers to give theauto-correlation matrix

R_(n)R*_(n)

. If a receiver-based normalisation was applied in step 7, the samenormalisation should be applied in step 8.

In step 9 the estimate of the n_(th) harmonic H_(n) emitted by thevibrator is determined. In principle H_(n) is given by H_(n)=

G_(n)R*_(n)

R_(n)R*_(n)

⁽⁻¹⁾. However, in practice the auto-correlation matrix will be badlyconditioned—that is the smallest eigenvalue will be much smaller thanthe largest eigenvalue, and taking an exact inverse runs the risk ofbeing dominated by noise. A standard singular value decomposition of

R_(n)R*_(n)

decomposes it into the product of three matrices, U, V, and Λ, where

R_(n)R*_(n)

=UΛV*, U and V are such that UU*=VV*=I where I is the identity matrix)and Λ is diagonal and real. U and V are the matrices of right and lefteigenvectors respectively, Λ is composed of the eigenvalues. The trueinverse matrix is given by

VΛ⁻¹U* The estimate of H_(n) is given by H_(n)=

G_(n)R*_(n)

(VLU*) where L is a diagonal matrix, identical to Λ⁽⁻¹⁾, except thesmallest elements have been replaced by zeros. One way of deciding howmany eigenvalues to retain and how many to set to zero comes fromcomparing the root-mean-square (rms) size of the elements of

R_(n)R*_(n)

. The rms size of the elements of VLU* times the rms size of

R_(n)R*_(n)

should be of order 1. Often only one eigenvalue is necessary. H_(n) isan N by M matrix for each frequency and set of shots.

At step 10 the matrix H_(n) is Fourier transformed back to the timedomain to obtain h_(n). The Fourier transform is carried out for a timewindow around T=0 using a smooth taper, and having a half width ofaround 0.25 seconds or less.

At step 11, h_(n) is convolved with the harmonic sweep used in step 4.The result of this convolution is the estimate of the vibrator outputfor the n^(th) harmonic. This is an estimate of the noise that appearson the data acquired during the preceding shot. The data acquired forthe preceding shot can be corrected for the harmonic noise, for exampleby subtracting the harmonic noise estimated from the raw data acquiredat the receiver. Alternatively, if the deconvolution stage is performedon correlated data, the harmonic noise can be correlated with theappropriate pilot sweep and then subtracted from the appropriatecorrelated records.

The above steps may be repeated for each harmonic which it is desired toremove. Typically, the second and third harmonics have the greatestamplitude, so that removal of just these harmonics may be sufficient.

At step 12 the shot records, which have now had the harmonic noiseremoved, are separated into their individual shot point components. Thisstep may be done on either uncorrelated or correlated shot records.Alternatively, the separation stage can be carried out on the summednoise estimates alone, and these separated noise estimates are thensubtracted from previous calculated individual shot point components.

The above method has been described with reference to the simultaneoussub-sweep acquisition method of FIG. 3. It may alternatively be appliedto a conventional slip-sweep acquisition method of the type shown inFIG. 2. The method can be simplified when applied to such a conventionalslip-sweep technique, since the cross-correlation and theauto-correlation are not matrixes, but are a number at each frequency.Thus, in step 9 the true inverse of the auto correlation can be used.

Use of this method requires a certain minimum time between sweeps. Inorder to remove the n^(th) harmonic, the minimum time betweenconsecutive sweeps is given by:

$\begin{matrix}{T_{\min} = \frac{\left( {n - 1} \right)f_{1}S}{n\left( {f_{1} - f_{0}} \right)}} & (1)\end{matrix}$

This assumes that the fundamental sweep varies linearly betweenfrequency f₀ and frequency f₁ and has a sweep time of S.

Thus, to remove the second harmonic the slip between consecutive sweeps(for example, T1−T0 in FIG. 2 or FIG. 3) must be slightly more than halfthe overall sweep duration, to remove the third harmonic it must beslightly more than two thirds of the sweep time, etc.

FIGS. 5 and 6 illustrate results of the method of FIG. 4. FIG. 5 showspart of a data record acquired at a receiver during simultaneousactuation of three seismic vibrators, with each vibrator being actuatedfour times. Although the data was acquired with a conventionalsimultaneous acquisition technique, rather than a simultaneousslip-sweep acquisition technique, the records were summed to simulatedata acquired by a simultaneous slip-sweep technique of the presentinvention. It will be seen that there is high frequency harmonic noiseacross much of the data. The twelve traces shown in FIG. 5 were acquiredat twelve separate receiver locations.

FIG. 6 illustrates the result of applying a harmonic estimation andremoval method of the type described with reference to FIG. 4 to thesecond and third harmonics in the data of FIG. 5. It can be seen thatthe high frequency harmonic noise has been significantly reduced.

An alternative technique for removing harmonic energy will now bedescribed. In contrast to the above-described method of harmonic noisereduction, which relies on being able to estimate the harmonic noise,the technique described below requires very little knowledge of theharmonic energy. The method requires only knowledge of at which timedifferent orders of harmonic energy arrives at a receiver. This methodmay be applied to both the simultaneous slip-sweep acquisition of FIG. 3or to conventional slip-sweep acquisition of seismic data. Onerestriction on this technique is that there must be at least one moresweep made than the number of shot points that are being separated.Thus, for standard slip-sweep data acquisition each vibrator must beactuated at least twice in each shot location. In a simultaneousslip-sweep acquisition technique in which each group includes twovibrators, at least three sweeps must be made in each location.

Consider initially the case of a standard slip-sweep acquisitiontechnique in which each vibrator group makes two shots in each location.After cross-correlation or deconvolution the signal components of thedata record for one shot will be the same as the signal component of thedata record for the other shots. If the harmonic noise component in thetwo records can be arranged to differ, then it is possible to reduce oreliminate the harmonic noise by combining the two records appropriately.One way of doing this that has been proposed previously is to vary thephase of the shots. If the phase of the n^(th) harmonic is n timesgreater than the phase of the fundamental sweep (which is usuallyapproximately true), then by choosing an appropriate phase differenceand summing the records after cross-correlation, one or more harmonicswill cancel out. If the two shots are 90° out of phase then the thirdharmonic will cancel. If the two shots are 180° out of phase then thesecond and fourth harmonics will cancel. If there are three shots, eachbeing 120° out of phase with the other two shots, then the second andthird harmonics will cancel.

The method described below uses a combination of an acquisitiontechnique and a processing technique, and does not depend on anyalgebraic phase relation.

The method described below is based on the principle that, if harmonicnoise on one shot appears at a different time from harmonic noise onanother shot, then appropriate stacking can eliminate or substantiallyreduce the noise. The stacking method that is used is time-frequencydiversity stacking. This stacking method works well when the signalcomponent of the two records is the same, but the noise appears indifferent locations in the time-frequency domain.

To make harmonic noise appear at different times in different shots, onemethod is to vary the delay between shots. The chosen time differencebetween a shot in one location and the subsequent shot (at anotherlocation) should differ from the time delay between another shot made inthe first location and subsequent shot thereto such that, aftertime-frequency decomposition, the largest peaks in the noise do notsubstantially overlay one another. The exact time delay that is requiredwill depend on the sweep rate, but it has been found that a differenceof between 1 and 2 seconds is generally sufficient if the sweep time isless than 10 seconds.

In a variation of this technique the effect of the time delay betweenshots is mimicked using multiple vibrator groups where the noise isarranged to come from shots from different, widely separated vibratorgroups. The spatial separation between the vibrator groups will induce atime delay so that, for most receivers, harmonic noise will arrive atdifferent times and time-frequency diversity stacking will be effectiveat removing the harmonic noise. For some receivers, the peaks in thenoise from different shots will overly one another, but this noise canbe removed during stacking using the technique disclosed in UK patentapplication No 2 359 363.

In the case of N vibrators, it is necessary to make at least (N+1) shotsof encoded sweeps, from which the n individual shot records can beextracted. For example, consider two groups of two vibrators each with asweep time S of 8 seconds and a listening time L of 5 seconds. Thefastest that these vibrators can perform three shots for each group isas follows:

TABLE 1 Group 1 shot 1 starts 0 seconds Group 2 shot 1 starts 5 seconds5 second time slip Group 1 shot 1 ends 8 seconds Group 2 shot 1 ends 13seconds 5 second slip time Group 1 shot 2 starts 10 seconds Group 2 shot2 starts 15 seconds 5 second slip time Group 1 shot 2 ends 18 secondsGroup 2 shot 2 ends 23 seconds 5 second slip time Group 1 shot 3 starts20 seconds Group 2 shot 3 starts 25 seconds 5 second slip time Group 1shot 3 ends 28 seconds Group 2 shot 3 ends 33 seconds 5 second slip time

Adding delays between the shots made by the first group and the shotthat follows each shot of the first group enables the harmonic noise tobe placed at different times in the shot records. For example, the aboveshot scheme can be modified by increasing the delay between a shot ofthe first group and the corresponding shot of the second group, asfollows:

TABLE 2 Group 1 shot 1 starts 0 seconds Group 2 shot 1 starts 5 seconds5 second time slip Group 1 shot 1 ends 8 seconds Group 2 shot 1 ends 13seconds 5 second slip time Group 1 shot 2 starts 10 seconds Group 2 shot2 starts 16 seconds 6 second slip time Group 1 shot 2 ends 18 secondsGroup 2 shot 2 ends 24 seconds 6 second slip time Group 1 shot 3 starts22 seconds Group 2 shot 3 starts 29 seconds 7 second slip time Group 1shot 3 ends 30 seconds Group 2 shot 3 ends 37 seconds 7 second slip time

In this modified shot scheme of Table 2, the varying delay between ashot of the first group and the corresponding shot of the second groupmeans that harmonic noise will appear at different times in the shotrecords. It is therefore possible to eliminate the harmonic noise usingthe “diversity simultaneous inversion technique” disclosed in GB 2 359363.

FIG. 7 illustrates the results of applying the diversity simultaneousinversion technique to the data shown in FIG. 5. It will again be seenthat a considerable reduction in harmonic noise has been provided.

In the shot scheme of table 2, the time delay between a shot of thefirst vibrator group and the corresponding shot of the second vibratorgroup increases with the shot number. The invention is not limited tothis, and the time delay between a shot of the first vibrator group andthe corresponding shot of the second vibrator group could alternativelydecrease with the shot number.

The technique of diversity simultaneous inversion may be combined withharmonic estimation and removal for those shots where the time delaysatisfies equation (1) above. For example, if the vibrator sweep is from10 Hz to 100 Hz, then the second harmonic can be estimated and removedif the slip time is greater than 4.5 seconds, the third harmonic can beestimated and removed if the slip time is more than 6 seconds, and thefourth harmonic can be removed if the slip time is more than 6.7seconds. Thus, for the acquisition scheme of Table 2, the secondharmonic may be estimated and removed from the harmonic noise in allshots, the third harmonic may be estimated and removed from the noise inthe second and third shot of each group, and the fourth harmonic may beestimated and removed from the noise in the third shot of each group.

FIG. 8 illustrates the results of applying both the harmonic estimationand removal technique and the diversity simultaneous inversiontechnique. FIG. 8 shows the results of carrying out the diversitysimultaneous inversion technique to the data shown in FIG. 6—which hasalready undergone a harmonic estimation and removal technique for thesecond and third harmonics. It will be seen that subsequent applicationof the diversity simultaneous inversion technique has resulted infurther reduction in noise.

The manner in which the above techniques are used depends on factorssuch as the number of groups, the time required to move a vibrator fromone desired shooting position to another, and the ratio of the totalvibrator sweep time to the listen time.

If the listen time is equal to the total sweep time divided by thenumber of vibrators in a group, then an efficient implementation is tohave two groups. At any one time, one group can be being re-positionedwhile the other group is acquiring data. The group that is acquiringdata shoots the same number of shots as there are vibrators in thegroup. Each vibrator sweeps nearly continuously, with only a short pauseat the end of each sweep to reset the equipment. When the first grouphas finished shooting, the second group is in position, and beginsshooting while the first group moves up to the next location. The sliptime is 100% of the sweep time, so harmonic estimation and removal maybe applied to the acquired data. However, as the number of shots isequal to the number of vibrators in each group, diversity-simultaneousinversion is not possible.

If, in another example, the sweep time for each vibrator is around twicethe listening time, then a method using three groups can usefully beemployed. In the case of nine vibrators, it will be possible to havethree groups of three vibrators, with vibrators in each group beingactuated four time with varying delays between shots. The shootingpattern would be arranged so that the first two shots of each groupalternate with the last two shots of the preceding group and so that thethird and fourth shots of each group alternate with the first two shotsof the subsequent group. For those shots where equation (1) issatisfied, harmonic noise can be estimated and removed. Diversitysimultaneous inversion may then be applied to the acquired data for eachgroup.

FIG. 9( a) is a schematic illustration of a seismic surveyingarrangement according to an embodiment of the invention. The seismicsurveying arrangement comprises a plurality of vibrators 1,2,3,4 and acontrol means 5. Two groups of vibrators A, B are defined in FIG. 9( a),with each group containing two vibrators in each group, but the seismicsurveying arrangement of the invention is not limited to this particularnumber of groups or to this number of vibrators in a group. The controlmeans 5 is able to actuate each group independently of the other group.For example, the control means may be electrically connected to eachgroup of vibrators so that it can send an electrical signal to aselected group to actuate each vibrator in the selected group. Thecontrol means is adapted to actuate the vibrators according to, forexample, a “simultaneous slip-sweep” method of the type described withreference to FIG. 3 or a “varying time delay” method of the typedescribed with reference to Table 2.

Four vibrators are shown in FIG. 9( a), arranged into two groups eachcontaining two vibrators but a seismic surveying arrangement theinvention is not limited to these numbers of vibrators and groups. Aseismic surveying arrangement of the invention for use with the“simultaneous slip-sweep” method may contain two or more independentlyactuable groups of vibrators, with at least one group containing morethan one vibrator. A seismic surveying arrangement of the invention foruse with the “varying time delay” method may contain two or moreindependently actuable groups of vibrators, or alternatively may containtwo or more independently actuable vibrators.

The seismic surveying arrangement of FIG. 9( a) further comprises anarray of one or more seismic receivers (two receivers 6,7 are shown inFIG. 9( a), but the seismic surveying arrangement is not limited to tworeceivers).

FIG. 9( b) is a schematic block diagram of the control means 5. Thecontrol means comprises a programmable data processor 8 with a programmemory 9, for instance in the form of a read only memory ROM, storing aprogram for controlling the control means 5 to actuate the vibrators1,2,3,4 according to, for example a method as illustrated in FIG. 3 orTable 2 or as defined by equation (1) above. The system furthercomprises non-volatile read/write memory 10 for storing, for example,any data which must be retained in the absence of power supply. A“working” or “scratchpad” memory for the data processor is provided by arandom access memory (RAM) 11. An input interface 12 is provided, forinstance for receiving commands and data. An output interface 13 isprovided, for instance for outputting actuation signals to a selectedreceiver or to a selected group of receivers. A program defining theactuation sequence of the receivers or receiver groups may be suppliedvia the input interface 12 or may alternatively be provided by amachine-readable store 14.

The program for operating the control means and for performing themethod described hereinbefore is stored in the program memory 9, whichmay be embodied as a semi-conductor memory, for instance of thewell-known ROM type. However, the program may be stored in any othersuitable storage medium, such as magnetic data carrier 9 a (such as a“floppy disc”) or CD-ROM 9 b.

The present invention is applicable to both land-based vibrator sourcesof seismic energy and the marine vibrator sources of seismic energy.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

1.-8. (canceled)
 9. A method of seismic survey comprising: actuating afirst vibrator group to start a first sweep at time T0; and actuating asecond vibrator group to start a second sweep at time T1, whereinT0<T2<T0+S1+L, where S1 is a sweep time of the first vibrator group andL is a listening time; wherein the time between the first sweep and thesecond sweep is T1−T0≧((n−1)*f₁*S1)/(n*(f₁−f₀)) where n is a naturalnumber, f₀ is the lower frequency limit of the vibrator sweep and f₁ isthe upper frequency limit of the vibrator sweep.
 10. The method asclaimed in claim 9, wherein at least one of the first vibrator group andthe second vibrator group comprises at least two vibrators.
 11. Themethod as claimed in claim 9, further comprising actuating the vibratorgroups at least as many times as there are vibrators in the respectivegroup.
 12. The method as claimed in claim 9, wherein: N is the number ofvibrators in one of the first vibrator group and the second vibratorgroup, whichever has the most vibrators; and actuating the vibratorgroups to make at least N+1 sweeps.
 13. The method as claimed in claim12, further comprising providing a phase change between sweeps.
 14. Themethod as claimed in claim 12, further comprising varying the delaytime, T1−T0, between a first sweep and a subsequent sweep.
 15. Themethod as claimed in claim 9, comprising actuating a third vibratorgroup at time T2, where T1<T2<T1+S2+L, where S2 is a sweep time of thesecond vibrator group.
 16. The method as claimed in claim 9, furthercomprising recording an acquired data from which it is possible toestimate a harmonic noise.
 17. The method as claimed in claim 9, furthercomprising recording an acquired data from which it is possible toestimate a harmonic contribution to a preceding shot by convolving anearth response, as determined from a content of a fundamental sweep,with a harmonic output a vibrator of one or more of the first vibratorgroup and the second vibrator group.
 18. The method as claimed in claim9, further comprising recording an acquired data from which it ispossible to estimate a harmonic noise.
 19. The method as claimed inclaim 9, further comprising recording an acquired data from which it ispossible to estimate a harmonic noise by cross correlating the acquireddata with the harmonic sweep.
 20. The method as claimed in claim 9,further comprising estimating from the acquired data the noise in theacquired data relating to the m^(th) harmonic, where m≦n.
 21. The methodas claimed in claim 9, further comprising providing a phase changebetween the first sweep and the second sweep.
 22. The method as claimedin claim 9, wherein T1−T0>L.
 23. The method as claimed in claim 9,wherein at least one vibrator is in both the first vibrator group andthe second vibrator group.
 24. A seismic surveying arrangementcomprising: a plurality of vibrators; and a control means adapted toinitiate a first sweep of a first vibrator group at time TO and toinitiate a second sweep of a second vibrator group at time T1, whereT0<T1<T0+S1+L where S1 is the sweep time of the first vibrator group andL is the listening time; the control means is further adapted toinitiate the first sweep and the second sweep such thatT1−T0≧((n−1)*S1*f₁)/(n*(f₁−f₀)), where n is a natural number, f₀ is thelower frequency limit of the vibrator sweep and f₁ is the upperfrequency limit of the vibrator sweep; and at least one of the firstvibrator group and the second vibrator group has more than one vibrator.25. The arrangement as claimed in claim 24, further comprising thecontrol means is adapted to actuate the vibrator groups at least as manytimes as there are vibrators in the respective group.
 26. Thearrangement as claimed in claim 24, wherein: N is the number ofvibrators in one of the first vibrator group and the second vibratorgroup, whichever has the most vibrators; and the control means isadapted to actuate the vibrator groups to make at least N+1 shots. 27.The arrangement as claimed in claim 24, further comprising the controlmeans is adapted to vary the delay time, T1−T0, during subsequent sweepsequences.
 28. The arrangement as claimed in claim 24, furthercomprising the control means is adapted to actuate a third vibratorgroup at time T2, where T1<T2<T1+S2+L, where S2 is a sweep time of thesecond vibrator group.
 29. The arrangement as claimed in claim 24,further comprising the control means is adapted to provide a phasechange between the first sweep and the second sweep.
 30. The arrangementas claimed in claim 24, wherein T1−T0>L.
 31. The arrangement as claimedin claim 24, wherein at least one vibrator is in both the first vibratorgroup and the second vibrator group.
 32. A method of processing slipsweep seismic data comprising: grouping the slip sweep seismic data intoresponses of at least two groups of vibrators, wherein at least onegroup has at least two vibrator sources; generating impulse responsesfor each source in each vibrator group; estimating the harmonic noiseswith cross-correlating slip sweep seismic data with a harmonic frequencysweep; and removing the harmonic noises.
 33. The method of claim 32wherein the cross-correlated data is Fourier transformed.
 34. The methodof claim 32 wherein the cross-correlated data is time windowed.
 35. Themethod of claim 32 wherein the data is Fourier transformed and timewindowed.
 36. The method of claim 32 wherein the harmonic frequencysweep is the same as the fundamental frequency sweep, except the sweeprate is an integer multiple of the sweep rate of the fundamentalfrequency sweep.
 37. The method of claim 32, wherein the slip sweepseismic data are responses to K shots at a shot location of N vibratorsin M groups, where K, M, N are integers, K is at least N+1, furthercomprising varying the phase of the shots.
 38. A seismic data processingsystem, comprising: a processor; and a computer readable mediacontaining computer readable instructions, wherein the instructionscontrol the processor to perform the steps of: grouping the slip sweepseismic data into responses of at least two groups of vibrators, whereinat least one group has at least two vibrator sources; generating impulseresponses for each source in each vibrator group; estimating theharmonic noises with cross-correlating slip sweep seismic data with aharmonic frequency sweep; and removing the harmonic noises.